Posted by chadstrickland: ...if that guy with the head wound ever recovered he would have probably wanted revenge for being so messed up and stuff...

"Would have probably wanted revenge" is a long, long way from posing an imminent threat of death or serious bodily harm.

But let's weigh the risks anyway, and see if murder would have prevented that or other possible future harm. So, the murderer is imprisoned. Who will protect his family against the thousands of other violent criminal actors roaming the streets and alleys?